stringify

A property value should be any JavaScript value except for undefined. If you do something like this (albeit it is legal):

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var foo = { bar: undefined };

will leads to confusing code. Because when accessing the property value:

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console.log(foo.bar); // undefined

It will returns undefined. But you are not sure if it means the property exists or not or the value of the property is set to undefined. Therefore, you should do:

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var foo = { bar: null };

This indicates that the property is expected, and with the value of null.

You do be able to check the existence of a property by:

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Object.keys(foo); // ['bar']

foo.hasOwnProperty('bar'); // true

But more importantly, if you serialize the object with JSON.stringify, properties with undefined will be omitted:

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JSON.stringify({ bar: undefined }); // '{}'

According to JSON specification, a value can be a string in double quotes, or a number, or true or false or null, or an object or an array. undefined is not a valid JSON value.

null is fine:

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JSON.stringify({ bar: null }); // '{"bar":null}'

So, for the best practice, avoid assigning undefined to a property of an object. Use null to indicate the expected property without a value. This will increase portability when using JSON to serialize.